1. Field of the Invention
This invention relates to the field of borehole drilling, and particularly to systems and methods for controlling the direction of such drilling.
2. Description of the Related Art
Boreholes are drilled into the earth in the petroleum, gas, mining and construction industries. Drilling is accomplished by rotating a drill bit mounted to the end of a “drill string”; i.e., lengths of pipe that are assembled end-to-end between the drill bit and the earth's surface. The drill bit is typically made from three toothed cone-shaped structures mounted about a central bit axis, with each cone rotating about a respective axle. The drill bit is rotated about its central axis by either rotating the entire drill string, or by powering a “mud motor” coupled to the bit at the bottom end of the drill string. The cones are forced against the bottom of the borehole by the weight of the drill string, such that, as they rotate about their respective axles, they shatter the rock and thus “bore” as the bit is turned.
Boreholes are frequently drilled toward a particular target and thus is it necessary to repeatedly determine the drill bit's position. This is typically ascertained by placing an array of accelerometers and magnetometers near the bit, which measure the earth's gravity and magnetic fields, respectively. The outputs of these sensors are conveyed to the earth's surface and processed. From successive measurements made as the borehole is drilled, the bit's “present position” (PP) in three dimensions is determined.
Reaching a predetermined target requires the ability to control the direction of the drilling. This is often accomplished using a mud motor having a housing which is slightly bent, so that the drill bit is pointed in a direction which is not aligned with the drill string. To affect a change of direction, the driller first rotates the drill string such that the bend of the motor is oriented at a specific “toolface” angle (measured in a plane orthogonal to the plane containing the gravity vector (for “gravity toolface”) or earth magnetic vector (for “magnetic toolface”) and the motor's longitudinal axis). When power is applied to the motor, a curved path is drilled in the plane containing the longitudinal axes.
One drawback of this approach is known as “drill string wind-up”. As the mud motor attempts to rotate the drill bit in a clockwise direction, reaction torque causes the drill string to tend to rotate counter-clockwise, thus altering the toolface away from the desired direction. The driller must constantly observe the present toolface angle information, and apply additional clockwise rotation to the drill string to compensate for the reaction torque and to re-orient the motor to the desired toolface angle. This trial and error method results in numerous “dog leg” corrections being needed to follow a desired trajectory, which produces a choppy borehole and slows the drilling rate. Furthermore, the method requires the use of a mud motor, which, due to the hostile conditions under which it operates, must often be pulled and replaced.